1. Field of the Invention
The present invention relates to an anode in which an anode current collector is provided with an anode active material layer, and a battery using it.
2. Description of the Related Art
In recent years, in connection with high-performance and multi-function of mobile devices, high capacities of secondary batteries, the power source for the mobile devices have been desired earnestly. As a secondary battery which meets this demand, there is a lithium secondary battery. However, in the case of using cobalt acid lithium for a cathode and graphite for an anode, which is currently a typical mode for the lithium secondary batteries, the battery capacity is in a saturated state, and it is extremely difficult to greatly obtain a high capacity of the battery. Therefore, from old times, using metallic lithium (Li) for an anode has been considered. However, in order to put this anode to practical use, it is necessary to improve efficiency of precipitation dissolution of lithium and to control dendrite precipitation form.
Meanwhile, a high capacity anode using silicon (Si), germanium (Ge), tin (Sn) or the like has been actively considered recently. However, when charge and discharge are repeated, these anodes are pulverized and miniaturized due to significant expansion and shrinkage of an active material, current collecting characteristics are lowered, and dissolution reaction of an electrolytic solution is promoted due to an increased surface area, so that their cycle characteristics are extremely poor. Therefore, an anode wherein an active material layer is formed on a current collector by vapor-phase deposition method, liquid-phase deposition method, sintering method or the like has been considered (for example, refer to Japanese Unexamined Patent Application Publication No. H08-50922, Japanese Patent Publication No. 2948205, and Japanese Unexamined Patent Application Publication No. H11-135115). When such an anode is used, miniaturization can be inhibited compared to conventional coating type anodes which are coated with a slurry containing a particulate active material, a binder and the like, and the current collector and the active material layer can be integrated. Therefore, electronic conductivity in the anode becomes extremely excellent, and high performance in terms of capacity and cycle life is expected. In addition, a conductive material, a binder, voids and the like which have conventionally existed in the anode can be reduced or excluded. Therefore, the anode can become a thin film essentially.
However, even when such an anode is used, sufficient cycle characteristics cannot be obtained, since there is a problem such as separation between the current collector and the active material and generation of crinkling in the current collector due to expansion and shrinkage of the active material associated with charge and discharge. Further, reactivity to an electrolyte is still high. There is also a problem that the reaction with the electrolyte associated with charge and discharge induces deterioration of the capacity.
As a method to solve these problems, for example, it is thinkable that an interlayer is formed between the current collector and the active material layer to improve contact characteristics between the current collector and the active material layer. To date, there have been reports about, for example, an anode in which a current collector is made of a metal or an alloy having a high mechanical strength and an interlayer made of copper (Cu) which is alloyed with an active material is formed between the current collector and the active material layer (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-83594) and an anode in which an interlayer containing molybdenum (Mo) or tungsten (W) is formed between a current collector and an active material layer (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-373644).
However, in the case where the interlayer made of copper or the like which is alloyed with an active material layer is formed between the current collector and the active material layer as described in Japanese Unexamined Patent Application Publication No. 2002-83594, there has been a problem that though contact characteristics between the current collector and the active material layer is improved, such effects are small compared to in conventional anodes in which a current collector is made of copper. Further, in the case where the interlayer containing molybdenum or tungsten is provided as described in Japanese Unexamined Patent Application Publication No. 2002-373644, there has been a problem that though excessive diffusion of a component element for the current collector into the active material layer can be inhibited, flexibility in a joint face between the current collector and the active material layer is often lost due to formation of such a metal layer having high hardness as an interlayer. Therefore, cycle characteristics have been hard to be improved.
Further, in Japanese Unexamined Patent Application Publication No. 2002-83594, tensile strength of the current collector is also described. However, as mechanical characteristics required for the current collector, flexibility such as elastic deformation ability is also important in addition to the strength. Therefore, it has been hard to improve cycle characteristics only by improving the tensile strength.